This invention is concerned with the integral deposition of vapor permeable polymeric films in and on a flexible substrate. More particularly, it is concerned with improved procedures for depositing the grain layer on substrates so as to produce products which are useful as replacements for natural leather in a variety of applications. In preferred embodiments of the invention the polymeric film is deposited on fibrous substrates.
Major advances have been made in recent years in the production of leather replacement products . . . so called artificial leathers. These products are now available in a large number of grades which are suitable for use as shoe uppers, garments, upholstery coverings, handbags, and the like. Many of these products comprise a thin, elastomeric, breathable film on a flexible fibrous substrate. Normally the film is deposited on the substrate by precipitation from an organic solution by the addition of a liquid which is miscible with the solvent, but which does not dissolve the polymer.
The process of precipitating polymer molecules from solution by introducing a non-solvent liquid for the polymer, which is miscible with the solvent of the solution, is well known. However, for the production of leather-like products it is desirable that the polymer deposit from its solution as a uniform microporous film which is substantially free of micro-voids. Microporous films, as the term is used in the art and as it is used in this application, are cellular films in which cells are so small that they are generally not visible even at 100 .times. magnification. Macro-voids are normally visible to the naked eye and their presence in films which are components of artificial leathers will have deleterious effects on aesthetic properties and on physical properties such as flex, tear strength, and the like, in the finished product.
A number of procedures have been devised in attempts to deposit microporous films which are substantially free of macro-voids. None of them are completely satisfactory. The best known of these methods are:
1. Exposing the wet film of polymer solution in an organic solvent to a humid atmosphere, e.g. a wet film of a dimethyl formamide solution of a polyurethane elastomer to an aqueous atmosphere having a controlled relative humidity. This is followed by bathing in a non-solvent such as water.
2. Adding water or other non-solvent to the organic solution of the polymer to convert the solution to a colloidal dispersion. The amount of non-solvent added is carefully controlled so that the polymer does not form a gel. The suspension is coated on the substrate as a wet film and deposition is completed by bathing in a non-solvent bath.
3. Adding a sufficient amount of water or other non-solvent to the polymer solution so as to form a gel. The gel is separated and coated on the substrate. Deposition is completed by bathing the coated substrate in a non-solvent bath.
4. Forming a wet film of polymer solution on a substrate and thereafter bathing the coated substrate with a mixture of an organic solvent for the polymer and a non-solvent for the polymer which is at least partially miscible with the organic solvent.
All of these methods are followed by a step in which the product is washed in additional non-solvent so as to remove substantially all of the solvent. The product is then dried.
These procedures are capable of producing microporous films substantially free of macro-voids, but the procedures are not free of drawbacks. The principal disadvantages are these:
1. The first method requires a long processing time for the deposition of a suitable film, and this adds to the cost of the procedure. Expensive air conditioning equipment is also necessary in order to control the relative humidity.
2. With the second method, the determination of the end point between the formation of the colloidal dispersion and the gel point is often quite difficult especially because of the tendency of the end point to vary appreciably from batch to batch of what are, apparently, the same polymers. This often leads to the production of final products with less than optimum physical properties. In extreme cases the solution may even be rendered useless.
3. With the third method the necessity of separating the gel adds to the cost of the procedure. Additionally, the viscosity of the gel is often so high as to cause difficulties in the coating operation.
4. In the fourth method the relatively high quantity of organic solvent which is employed often causes deleterious effects on the substrate by softening or even dissolving some of its components. Additionally the use of such relatively large quantities of organic solvent increases the cost of the operation and may cause operational hazzards.
In view of these difficulties the art has long sought a facile, economic procedure for the preparation of microporous, elastomeric films suitable as grain layers in leather replacement products. This invention provides such a method.
Before describing the invention in more detail it will be convenient to define certain of the terms which will be employed.
Fiber:
Natural and synthetic materials of suitable denier, length and other dimensions such as polyesters, acrylics, polyamides, modacrylics, vinyls, cellulosics, wool, silk, etc. Inorganic fibers such as glass are included, but the preferred fibers for the preparation of leather-like compositions are normally organic fibers. They can be polyamides, such as polyhexamethylene adipamide (nylon 66) or polycaporamide (nylon 6); polyesters, such as polyethylene terephthalate or polydimethylcyclohexyl terephthalate; acrylics such as polyacrylonitrile; vinyls, such as polyvinyl chloride or polyvinyl alcohol; cellulosics such as rayon, etc., and wool. Mixtures of two or more fiber types may be employed.
As used herein, the term "fiber" includes tow, staple, continuous filament and similar fiber forms. The fibers may be present as yarns. They may be crimped (whether or not heat-set) or uncrimped. For the preparation of various substrates fibers employed may generally have a denier between about 0.5 and 6 and preferably between 0.5 and 3. Fiber lengths of at least about 1/2 inch are desirable. Fibers in conventional textile lengths, e.g. up to three or more inches, are generally suitable for the preparation of fibrous substrates.
As used herein "fiber" refers to a product which is at least 500 times as long as it is wide. This will distinguish fibers from particles which normally do not have any one dimension appreciably greater than another dimension, and are often essentially spherical.
Fleece:
The structure formed by processing the fibers in the appropriate equipment including, for example, carding, cross-layering, air-laying, etc. Excellent substrates for use in this invention are prepared from isotropic fleeces such as may be formed on air-lay equipment. However, a cross-laid fleece in conjunction with other directional structures such as carded fleece, scrim, warp yarn, and the like, can also be employed to prepare substrates. Isotropic continuous filament structures are also suitable.
Foam:
Relatively low density, porous, cellular, flexible, resilient materials. Polyurethane foam is preferred for the preparation of substrates, although rubber latex, vinyl foams and other foams having properties similar to polyurethane foams may also be used.
Web:
The product formed by combining the fleece and the foam as by needling the fleece into the foam. Reference will also be made to composite webs in which there are at least two fleece components.
Substrate:
The product formed by depositing an elastomer throughout the web. Composite substrates are formed from composite webs by elastomeric deposition.
The term "substrate" is used herein in two different senses. It is employed in the generic sense to refer to any base which is used in association with the grain layer of the invention. It is also used in the more restricted sense of the three component fiber-foam-elastomeric filler substrate which is defined in the previous paragraph. It is not believed that this will cause any confusion.
Grain layer:
The term "grain layer" is used in association with the description of this invention to describe that portion of the total structure which is analogous to the grain layer of natural leather. The grain layer may be prepared from the same elastomers used to prepare the substrates of composite substrates.
Solvent:
The liquid used to dissolve the polymer which will deposit, precipitate or coagulate to form the grain layer.
Non-solvent:
The liquid which is used to deposit the polymer from the solution. It may be water or an aqueous blend. It is normally completely miscible with the solvent. It is not necessary that the polymer be totally insoluble in the non-solvent.
Wetting liquid:
This is the liquid used to wet the substrate. Normally it will be identical with the non-solvent but this is not necessarily so. It should have substantially the same solubility characteristics with respect to the polymer, and the same miscibility characteristics with respect to the solvent, as does the non-solvent.
The foregoing rather specialized definitions are especially useful in defining the preferred leather replacement products which can be prepared in accordance with this invention in which an elastomeric film is deposited as a grain layer in the upper strata and above the surface of a fibrous substrate comprising randomly oriented fibers entangled and interlocked with each other.